Find your filter size
Type the old filter’s measured width, height and depth. Fractions work: 15 1/2, 15-1/2 or 15.5.
*Typical actual size follows one manufacturer’s published convention — about 0.5″ under on width and height, and 0.75 / 1.75 / 3.63 / 4.75″ deep for 1 / 2 / 4 / 5″ filters (Filterbuy size chart, checked 2026-10-09). Filterbuy itself says this is its convention, not an industry standard. Read the actual size on the frame of the filter you are replacing.
How to Measure Your Filter
- Remove the existing filter from its slot
- Look for the nominal size printed on the frame (e.g. 16x25x1)
- If no size is printed, measure length, width, and depth with a tape measure
- Round up to the next whole inch — at the brands checked for this page the actual panel is about half an inch smaller than the label on width and height
- The first number is width, the second is height, and the third is depth
Air filter nominal vs actual size: what is the difference?
The nominal size is the number printed on the frame and the one you buy by. The actual panel is cut slightly smaller so it slides into the track without binding, so a filter labelled one size measures a fraction of an inch less on each face.
That is why a filter can be labelled the same as the one you removed and still sit loose or refuse to go in: the actual dimensions are a convention rather than a standard, and manufacturers set their own tolerances. The chart on this page lists the common actual sizes against each nominal label.
Two practical consequences. First, measure the slot, not the old filter, if the old one was a poor fit β a filter that is too small lets air bypass it entirely, which is worse than a lower MERV rating. Second, when you find a brand whose actual dimensions fit your track, note it; switching brands at the same nominal size can change the fit.
Common Air Filter Sizes
Most residential HVAC systems use one of these standard filter sizes. Sizes are listed as Width x Height x Depth (inches). The nominal size is what you buy by; the actual panel is cut slightly smaller so it slides into the track.
The actual sizes below are the common convention, not a standard. Manufacturers set their own tolerances, so the offset is not identical across brands or depths, and thicker filters do not follow the same reduction as 1-inch panels. Before you buy, read the actual size printed on the frame of the filter you are replacing, or measure it β a filter that is a quarter-inch too wide will not seat, and one that is too small lets air bypass it.
| Size (W x H x D) | Actual Size (typical) | Common Use | Availability |
|---|
MERV Rating Guide
MERV (Minimum Efficiency Reporting Value) rates a filter's ability to capture particles. Higher MERV = finer filtration but more airflow restriction.
The EPA’s table below gives the minimum average share of particles each rating must trap in three size bands, from 0.3 to 10 microns. For homes the EPA advises: “choose a filter with at least a MERV 13 rating, or as high a rating as your system fan and filter slot can accommodate.” (EPA, What is a MERV rating?, updated January 14, 2026.)
| MERV | 0.3–1.0 µm | 1.0–3.0 µm | 3.0–10 µm |
|---|
Filter Type Comparison
Choose the right filter type based on your needs, budget, and air quality goals.
Side-by-Side Comparison
| Feature | Fiberglass | Pleated | Electrostatic | HEPA |
|---|---|---|---|---|
| Upfront cost | Lowest | Low to moderate | Higher (reusable) | Highest |
| Rating | Low MERV, check label | Wide MERV range, check label | Check label | Not on the MERV scale; HEPA means ≥99.97% at 0.3 µm (EPA) |
| Replacement | Per label | Per label | Wash per maker’s instructions | Per label |
| Fits a standard 1″ furnace slot? | Yes | Yes | Usually | Ask your HVAC installer first |
| Airflow resistance | Minimal | Rises with MERV | Low to moderate | High |
Earlier versions of this table gave prices, lifespans in days and a “MERV 17–20” band for HEPA. We removed them: we could not trace the prices and lifespans to a source, and the EPA states HEPA values are not based on the MERV system.
Replacement Reminder
There is no single right interval: it is printed on the filter packaging and depends on the filter. ENERGY STAR’s maintenance checklist says to “inspect, clean, or change air filters once a month” in a central air conditioner, furnace or heat pump (ENERGY STAR). This tool combines the two: a monthly look, and a change date from the label.
Pets, smoking, renovation dust or a filter that looks grey at the monthly check are reasons to change sooner than the label — the look decides, not a formula.
Further reading
Complete air filter size guide with common sizes, MERV rating explainer, filter type comparison, replacement schedule calculator, and size finder tool.
Related tools
Sources
Filter ratings and how much a home air cleaner can actually do are covered in the EPA's residential air cleaner guidance.
- US EPA β Air Cleaners and Air Filters in the Home
- US EPA β What is a MERV rating? (updated 2026-01-14): the MERV 1–16 particle-band table, the MERV 13 home recommendation and the HEPA 99.97% at 0.30 µm definition used on this page.
- ENERGY STAR β Maintenance checklist: inspect, clean or change air filters once a month.
- Filterbuy β Filter size chart: nominal vs actual dimensions (0.5″ under on width and height; 0.75, 1.75, 3.63 and 4.75″ deep). The page calls this its own convention.
- Atomic Filters β Nominal vs actual sizes: 16x25x1 = 15.5 x 24.5 x 0.75, 20x20x1 = 19.5 x 19.5 x 0.75.
Links checked 2026-10-09. We are not affiliated with any filter seller and earn nothing from these links.
Frequently asked questions
Does a higher MERV always mean a better filter?
Not for every system. Higher MERV catches finer particles but adds airflow resistance. The EPA recommends at least MERV 13 for homes, or as high a rating as your system fan and filter slot can accommodate — so check what your equipment allows before fitting a higher one. HEPA is a separate standard, not a point on the MERV scale.
How long does each filter type last?
As long as the interval printed on its packaging, and no longer than a monthly check says. ENERGY STAR advises inspecting, cleaning or changing the filter once a month; the replacement reminder on this page turns the label interval into a change date and monthly check dates you can add to a calendar.
Is the cheapest filter false economy?
It depends what you want from it. Fiberglass is the cheapest and sits at the low end of the MERV scale — it is there to keep debris off the equipment more than to clean the air. A MERV 13 pleated filter traps at least half of 0.3–1 micron particles under the EPA table, which is why the EPA points homes toward it.
What if my filter size is not a standard one?
Measure the slot and use the size finder at the top of this page, which rounds your measurements up to the label size and shows whether it is a stock size. Most residential systems take one of the common sizes listed at the top, and buying a stock size is both cheaper and easier to replace than having one cut.